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Authors:
James F. Kurose, Keith W. Ross
Chapter:
Wireless And Mobile Networks
Exercise:
Problems
Question:11 | ISBN:9780132856201 | Edition: 6

Question

In Section 6.5, one proposed solution that allowed mobile users to maintain their IP addresses as they moved among foreign networks was to have a for- eign network advertise a highly specific route to the mobile user and use the existing routing infrastructure to propagate this information throughout thenetwork. We identified scalability as one concern. Suppose that when a mobile user moves from one network to another, the new foreign network advertises a specific route to the mobile user, and the old foreign network withdraws its route. Consider how routing information propagates in a distance-vector algorithm (particularly for the case of interdomain routing among networks that span the globe).

a. Will other routers be able to route datagrams immediately to the new for- eign network as soon as the foreign network begins advertising its route?

b. Is it possible for different routers to believe that different foreign networks contain the mobile user?

c. Discuss the timescale over which other routers in the network will eventu- ally learn the path to the mobile users.

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Answer

a) No. All the routers might not be able to route the datagram immediately. This is
because the Distance Vector algorithm (as well as the inter-AS routing protocols
like BGP) is decentralized and takes some time to terminate. So, during the time
when the algorithm is still running as a result of advertisements from the new
foreign network, some of the routers may not be able to route datagrams destined
to the mobile node.
b) Yes. This might happen when one of the nodes has just left a foreign network and
joined a new foreign network. In this situation, the routing entries from the old
foreign network might not have been completely withdrawn when the entries
from the new network are being propagated.
c) The time it takes for a router to learn a path to the mobile node depends on the
number of hops between the router and the edge router of the foreign network for
the node.

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